Argon and halogen geochemistry of hydrothermal fluids in the Loch Ainort granite, Isle of Skye, Scotland

Highly turbid alkali feldspars from the Loch Ainort granite (59 Ma), Isle of Skye, have been analysed using the ⁴⁰Ar-³⁹Ar method to obtain chronological and chemical (K, Cl, Br, I) information concerning their origin and hydrothermal alteration. Three methods of gas extraction have been applied to neutron-irradiated samples: laser probe spot fusion of feldspars, in vacuo crushing of a feldspar/quartz separate, and laser stepped heating of the crushed residue. Apparent ages obtained by laser probe spot fusion are mostly similar to the 59 Ma intrusion age. Analyses of relatively pristine regions give some high apparent ages (>59 Ma) indicating the presence of small amounts of ⁴⁰Ar_E (excess ⁴⁰Ar). Crushing releases significant amounts of ⁴⁰Ar_E, but is dominated by an ⁴⁰Ar_A (atmospheric ⁴⁰Ar) component. ⁸⁴Kr/³⁶Ar values obtained by crushing are higher than air and are consistent with air equilibration with fresh water at low temperature ∼ 20°C). Therefore, ⁴⁰Ar_A was most probably introduced as palaeoatmospheric argon dissolved in the circulating hydrothermal fluids that interacted with the granite, thus supporting a meteoric origin for the fluids. Stepped heating gives a flat age spectrum and an age of 56±4 Ma. Crushing and stepped heating both released significant amounts of halogens with high Br/Cl and I/Cl ratios; excess Xe is also present as indicated by the high ¹³²Xe/³⁶Ar values. It seems likely that the halogen (and possibly Xe) enrichments resulted from interaction of the meteoric fluids with Jurassic sedimentary country rocks.